Length & number of radials again
 

"Reg Edwards" wrote in message
...
Frank,

So NEC4 cannot calculate input impedance of the radial system and we
have almost reached a dead end.

Would it be possible to insert a loading coil ( 2.48 uH ) at the
bottom of the antenna to tune out its input reactance ( which is what
my program does.)

Then repeat the efficiency calculation and tell me what you get.
----
Reg.

Reg, According to NEC 4.1 the input impedance is near 40 ohms
(39.9373 + j 0.394926 ohms) at resonance (8.07 MHz). With
100 W input the total radiated power computes to 31.8 W. I am
continuing with checking the program to be certain I have not made
an error, and also working on a NEC solution to the input impedance
of one radial. Note that the computation also includes copper loss,
which should be insignificant. I have also included a copy of my
code below.

Frank

CM Reg's test Vertical
CE
GW 1 36 0 0 9 0 0 0.05
GC 0 0 .9 0.00082 0.00082
GW 38 3 0 0 0.05 0 0 -0.025 0.00082
GW 2 40 0 0 -0.025 0 10 -0.025
GC 0 0 1.11 0.00082 0.00082
GM 1 35 0 0 10 0 0 0 2 1 2 40
GE -1
GN 2 0 0 0 16 0.0067
FR 0 1 0 0 8.07 0.01
EX 0 38 2 00 89.37696044 0
LD 5 1 1 36 5.8001E7.......

........................

.........................
LD 5 36 1 60 5.8001E7
RP 0 181 1 1000 -90 90 1.00000 1.00000
EN

Length & number of radials again
 

Frank,
Thanks for information so far. I need time to study it.

Could you tell me the

efficiency,
antenna input resistance component
and resonant frequency,

using our standard set of input data, ie., 36 radials, 10 metres long,
when frequency is set exactly to its 1/4-wave resonant value around
8.3 MHz. Input reactance = zero or very few ohms.

Thanks.
----
Reg.

Length & number of radials again
 

Preliminary numbers from Frank's NEC-4 run on Reg's model below:

Caveat: I have not been able to ask Frank if the segments are all the same
length along the radial wire. The info below is based on that assumption.

The radial is 10 meters long, buried about 1 inch. I'm reading the numbers
from the graph that Frank sent me. The radial wire is 40 segments long or
..25 meters per segment. The antenna is 9 feet long and modeled at 8.07 mhz.

If I'm reading it right, at 30 segments along the radial wire, the current
has dropped from a peak of 0.6 amps to 0.2 amps. 30 segments seems to be 7.5
meters out. If the current is still 0.2 amps at 7.5 meters out on a 10 meter
radial, then Reg's approach fails. He indicated 20 dB down at a short
distance out. 75% of the way out on the 10 meter radial, the current is down
0.2/0.6 = .33. 10log * 0.33 = 4.8 dB (if I did that right).

So...it seems that the current along the radial is down only 4.8 dB at 75%
of it's length. Reg indicated that it should be down 20 dB at about 1/3 of
its length.

At the 35th segment of the radial, the current is 1/6th or 7.8 dB down. This
is at 90% of the radial's length.

At the 39th segment of the radio the current is .025 amps. 0.025/6 = .0146.
10log * .0146 = 14 dB down. That is only 14 dB down at 100% of the radial
length.

I'm using 10 log * (I1/I2) for for the dB calcs...I think current ratios and
power ratios are 10log, and voltage is 20log.

It is possible I'm interpreting Frank's graph incorrectly or applying the
attenuation that Reg refers to incorrectly. I'm just so glad to see some
numbers for current distribution along a radial wire from NEC-4, that I had
to post what I see.

Eyeballing it looks like this:(the radial wire starts at segment 39 and runs
to segment 79)

Segment 39 0.60 amps, distance from source = 0, dB = 0
Segment 49 0.54 amps, distance from source = 2.5 meters, dB = 0.46 dB
Segment 59 0.42 amps, distance from source = 5.0 meters, dB = 1.5 dB
Segment 69 0.22 amps, distance from source = 7.5 meters, dB = 4.3 dB
Segment 79 0.025 amps, distance from source = 10 meters, dB = 14.8 dB

What does Reg's program predict for dB down on this sample antenna?

Using 25 and 25 for soil and the info Frank gave me:

Reg's program shows radial attenuation of 20 dB at 2.3 meters from the
source.

Side by side with the NEC-4 data

Distance Reg NEC-4 (dB down)

2.5 m 21.2 0.46

5.0 m 42.4 1.5

7.5 m 63.9 4.3

10 m 83.3 14.8


These numbers are so far apart, it looks like I did something terribly
wrong. Someone please correct me.
Keep in mind these are preliminary attempts to analyze the NEC-4 based graph
that Frank sent me. I really do hope I did something wrong.

....hasan, N0AN

"Reg Edwards" wrote in message
...
Frank,

Just to confirm we are both working on the same system, I have -

Number of radials = 36
Length of radials = 10 m
Diameter of radials = 2 mm
Frequency = 7 MHz
Antenna height = 9 m
Antenna diameter = 1.64 mm = 14 AWG
Ground resistivity = 150 ohm-metres
Ground permittivity = 16

IMPORTANT:

If NEC4 gives you the input impedance of the radial system I should be
very pleased to know what it is.

Otherwise we shall have no idea where the discrepancy arises - in the
radial system or in the antenna efficiency calculation.

Radiating efficiency is estimated by my program by the well-known
formula -

Efficiency = Rrad / ( Rrad + Rradials )

provided antenna and radials reactance are tuned out.

Whereas NEC4 calculates efficiency by integrating power flow over a
hemisphere WITHOUT tuning out antenna and radials reactance.
Altogether different.
----
Reg, G4FGQ

Length & number of radials again
 

CAUTION CAUTION CAUTION:

The wire segments are NOT equal in this model. Frank is sending me a new one
with linear segments. I'll correct the errors below as soon as I get the new
values.

....hasan, N0AN

"hasan schiers" wrote in message
...
Preliminary numbers from Frank's NEC-4 run on Reg's model below:

Caveat: I have not been able to ask Frank if the segments are all the same
length along the radial wire. The info below is based on that assumption.

The radial is 10 meters long, buried about 1 inch. I'm reading the numbers
from the graph that Frank sent me. The radial wire is 40 segments long or
.25 meters per segment. The antenna is 9 feet long and modeled at 8.07
mhz.

If I'm reading it right, at 30 segments along the radial wire, the current
has dropped from a peak of 0.6 amps to 0.2 amps. 30 segments seems to be
7.5 meters out. If the current is still 0.2 amps at 7.5 meters out on a 10
meter radial, then Reg's approach fails. He indicated 20 dB down at a
short distance out. 75% of the way out on the 10 meter radial, the current
is down 0.2/0.6 = .33. 10log * 0.33 = 4.8 dB (if I did that right).

So...it seems that the current along the radial is down only 4.8 dB at 75%
of it's length. Reg indicated that it should be down 20 dB at about 1/3 of
its length.

At the 35th segment of the radial, the current is 1/6th or 7.8 dB down.
This is at 90% of the radial's length.

At the 39th segment of the radio the current is .025 amps. 0.025/6 =
.0146. 10log * .0146 = 14 dB down. That is only 14 dB down at 100% of the
radial length.

I'm using 10 log * (I1/I2) for for the dB calcs...I think current ratios
and power ratios are 10log, and voltage is 20log.

It is possible I'm interpreting Frank's graph incorrectly or applying the
attenuation that Reg refers to incorrectly. I'm just so glad to see some
numbers for current distribution along a radial wire from NEC-4, that I
had to post what I see.

Eyeballing it looks like this:(the radial wire starts at segment 39 and
runs to segment 79)

Segment 39 0.60 amps, distance from source = 0, dB = 0
Segment 49 0.54 amps, distance from source = 2.5 meters, dB = 0.46 dB
Segment 59 0.42 amps, distance from source = 5.0 meters, dB = 1.5 dB
Segment 69 0.22 amps, distance from source = 7.5 meters, dB = 4.3 dB
Segment 79 0.025 amps, distance from source = 10 meters, dB = 14.8 dB

What does Reg's program predict for dB down on this sample antenna?

Using 25 and 25 for soil and the info Frank gave me:

Reg's program shows radial attenuation of 20 dB at 2.3 meters from the
source.

Side by side with the NEC-4 data

Distance Reg NEC-4 (dB down)

2.5 m 21.2 0.46

5.0 m 42.4 1.5

7.5 m 63.9 4.3

10 m 83.3 14.8


These numbers are so far apart, it looks like I did something terribly
wrong. Someone please correct me.
Keep in mind these are preliminary attempts to analyze the NEC-4 based
graph that Frank sent me. I really do hope I did something wrong.

...hasan, N0AN

"Reg Edwards" wrote in message
...
Frank,

Just to confirm we are both working on the same system, I have -

Number of radials = 36
Length of radials = 10 m
Diameter of radials = 2 mm
Frequency = 7 MHz
Antenna height = 9 m
Antenna diameter = 1.64 mm = 14 AWG
Ground resistivity = 150 ohm-metres
Ground permittivity = 16

IMPORTANT:

If NEC4 gives you the input impedance of the radial system I should be
very pleased to know what it is.

Otherwise we shall have no idea where the discrepancy arises - in the
radial system or in the antenna efficiency calculation.

Radiating efficiency is estimated by my program by the well-known
formula -

Efficiency = Rrad / ( Rrad + Rradials )

provided antenna and radials reactance are tuned out.

Whereas NEC4 calculates efficiency by integrating power flow over a
hemisphere WITHOUT tuning out antenna and radials reactance.
Altogether different.
----
Reg, G4FGQ

Length & number of radials again
 

Frank,
Thanks for information so far. I need time to study it.

Could you tell me the

efficiency,
antenna input resistance component
and resonant frequency,

using our standard set of input data, ie., 36 radials, 10 metres long,
when frequency is set exactly to its 1/4-wave resonant value around
8.3 MHz. Input reactance = zero or very few ohms.

Reg'

With the above parameters; summarizing the data obtained so far:

Efficiency 31.8%;
Antenna input resistance component 36.21 - j 3.1, and;
The resonant frequency 8.07 MHz.

73,

Frank

Length & number of radials again
 

Frank,

Having used NEC4 to derive the input impedance of a single radial, it
is now in your hands to settle the discussion about attenuation along
radials and the distance at which a radial becomes ineffective. The
spectators are waiting!

We already have the input impedance of a single radial of length 10
metres at 7.0 MHz, with resistivity = 150 and permittivity = 16.

Using our standard set of data, I suggest you increase the SINGLE
radial length in increments of 3 metres until the input impedance Zin
stops changing and becomes relatively constant. That value of Zin
will be equal to Zo = Ro + jXo, the complex characteristic impedance
of the equivalent transmission line.

It might never become absolutely constant because NEC4 will take into
acount the effect of current flowing in the soil which, although it is
decreasing, eventually it will be substantially greater than that in a
long radial. ( My program does not do this.) But you should be able to
judge the distance at which radial attenuation is about 18 or 20dB,
ie., when Zin = Zo.

The question of efficiency is of less importance. It doesn't matter
what the efficiency is because you are using the antenna input
impedance plus radial input impedance only to deduce radial input
impedance in the same way as if you were measuring it. Be careful
with the signs of reactances. ;o)

At your leisure you may find a way how to do 36 and other numbers of
radials, at different frequencies. A 1/4-wave resonant antenna is
always best. The optimum length of a radial will decrease as
frequency increases. At 14 MHz the effect of permittivity kicks in
quite strongly. And with 120 or more radials you might be able to
demonstrate BL&E were quite correct when they concluded that a
virtually perfect ground. at MF, is independent of soil conditions.
----
Reg.

Length & number of radials again
 

Corrected numbers for linearly segemented radials from Frank's latest NEC-4
model of one buried radial wire, compared to Reg's program.

Side by side with the NEC-4 data


This is how many dB down the current is as you move outward from the origin
of the radial.

Distance Reg NEC-4 (dB down)

1.0 m 2.5 1.3

3.5 m 8.7 4.4

5.9 m 14.9 8.7

8.5 m 21.4 10.0

9.7 m 24.4 23.8

Conclusion: the current drop along the radial is no where near as fast as
Radials3 predicts, therefore shortening the radials as much as the program
shows will increase losses significantly.

I find it VERY interesting, that at the full length of 10m, there is good
agreement between Reg's program and NEC-4.

If I were going to base my conclusions on this preliminary small sample, I
would say that Reg's program does not hold up for short radials. BL&E, W8JI
and now NEC-4 all indicate that there is no where near 20 dB of attenuation
in short radials. To confirm this isn't an odd case, a lot more runs would
need to be done with varying lengths and radial numbers...but I have to say,
it ain't lookin' good for Radials3 in terms of fairly representing the
rapidity with which currents diminish on a radial wire over its length.

Bottom Line:

For the present, the articles in QST, ARRL Handbook, Low-Band DX'ing and
W8JI's findings are the ones I would follow. The first three are all the
same study and that formula is based on BL&E. The following data are from a
spreadsheet I used to calculate the optimum length and number of radials
based on the above sources. I put the BL&E data in the spreadsheet as a
reference. The numbers are how many dB down the field strength was for a
given number and length of radials.

Brn/Lw/Ep



# Rad 0.137 wl 0.274 wl 0.411 wl
2 -4.36 -4.36 -4.05
15 -2.40 -1.93 -1.65
30 -2.40 -1.44 -0.97
60 -2.00 -0.66 -0.42
113 -2.00 -0.51 0 (Ref)


Here are a few runs for 80 meters of various numbers and lengths of radials
that should be within a dB or so of optimum (BL&E).(Based on the references
noted above) 3.7 mhz, 1/4 wave vertical. The formula is based on tip
separation at the perimeter. Too much separation increases loss, too little
wastes wire. All based on wavelenthgs, of course. I believe the maximum tip
separation recommended was .015 wavelength.

Available Wire # of Radials Length of Radials
500' 25 19.7' (not within a dB, not
enough wire)
1000' 36 27.8' (not within a dB, not
enough wire)
1500' 44 34.0'
2000' 51 39.3'
3116' 63 49.0 (should be within 0.5 dB
of BL&E Optimum)

My final setup will be 46 radials 50' long. I have 26 right now. It looks
like for 50' long radials, I should really have 63 of them, otherwise, I
could have stayed at 51 radials only 39.3' long. All this says is that I'm
not making the "most" out of the available wire I had. (which makes sense,
given I've added radials over time, and didn't have a final plan).

At this point, it looks like when copper prices drop, I need to get another
850' of wire and put in 17 more radials and I will have met the criteria for
the formula. (Be within 0.5 dB of maximum field strength according to BL*E).

If anyone wants a copy of the Excel spreadsheet, just email me and I'll send
it to you as an attachment.
Only two variables should be entered: Total length of available wire and
Frequency in Mhz. Everything else is calcuated.( I did not protect any of
the fields, so if you enter data into a calculated field, you'll have to
reload your spreadsheet from a non-messed-up one...so save a virgin copy
somewhere until you protect the appropriate cells.)

73,

....hasan, N0AN

Length & number of radials again
 

Reg Edwards wrote:
Having used NEC4 to derive the input impedance of a single radial, it
is now in your hands to settle the discussion about attenuation along
radials and the distance at which a radial becomes ineffective. The
spectators are waiting!

Reg, how did you determine how much attenuation there is
in a radial because of the surrounding ground?
--
73, Cecil http://www.qsl.net/w5dxp

Length & number of radials again
 

Fellow Experimenters, Frank and Hasan.

I havn't the foggiest idea what you are doing with NEC4 but you should
be aware that, according to Radial_3, there are 3 resonant frequencies
with a single radial at lengths shorter than 10 metres and at a
frequency of 7 MHz.

The propagation velocity is very low. VF = 0.225

Funny unexpected things happen on multi-resonant lines especially when
Zo has a relatively large positive angle. Before you draw any
conclusions about deducing attenuation from your output data you
should take into account the line is -

1/4-wave resonant at 2.4 metres.
1/2-wave resonant at 4.8 metres.
3/4-wave resonant at 7.4 metres.

and at 10 metres it is very near to full-wave resonance. It can be
assumed the far end is open-circuit. Actually it isn't. It behaves
as if it is slightly longer.

It is significant that at 10 metres and 7 MHz, you have concluded that
the radial is about 20dB long. Which approximately agrees with my
program as being the length beyond which there is not much point in
extending it.

But the best way of determining attenuation is to do what I have
suggested - increase radial length in short increments and observe
what happens to radial input impedance. Eventually, Zin will converge
on Zo if it hasn't already done so. I should very much like to know
what Zo is and at what length it occurs. I have to assume NEC4 knows
what it's doing! ;o)
----
Reg.

Length & number of radials again
 

Reg, how did you determine how much attenuation there is
in a radial because of the surrounding ground?
--
73, Cecil
==========================================
Cec,

I don't have enough time left to write a thick book. But as an
engineer and radio amateur with 60 years (on and off) experience of
transmission lines ( from 0.05 Hz to 3 GHz ), and having once read
something about Oliver Heaviside's trouble with university professors,
I was able to make an intelligent guess.

It remains to be seen what the uncertainty is.

Isn't there anything in the Handbook or Google? ;o)
----
Yours, Reg.